The primary function of Tau is to bind to and stabilize microtubules, which are important structural components of the cytoskeleton involved in mitosis, cytokinesis, and vesicular transport. Tau is found in multiple tissues, but is particularly abundant in axons of neurons. In humans, there are six isoforms of Tau that are generated by alternative splicing of exons 2, 3, and 10. Splicing of exons 2 and 3 at the N-terminus of the protein leads to inclusion of zero, one, or two 29 amino acid acidic domains and is termed 0N, 1N, or 2N Tau respectively. The influence of these domains on Tau function is not fully clear, though may play a role in interactions with the plasma membrane. Inclusion of exon 10 at the C-terminus leads to inclusion of the microtubule binding domain encoded by exon 10. Since there are 3 microtubule binding domains elsewhere in Tau, this Tau isoform (with exon 10 included) is termed 4R Tau, where ‘R’ refers to the number of repeats of microtubule binding domains. Tau without exon 10 is termed 3R Tau. Since more microtubule binding domains (4R compared with 3R) increases the binding to microtubules, 4R Tau presumably significantly increases microtubule binding and assembly. The ratio of 3R/4R Tau is developmentally regulated, with fetal tissues expressing exclusively 3R Tau and adult human tissues expressing approximately equal levels of 3R/4R Tau. Deviations from the normal ratio of 3R/4R Tau are characteristic of neurodegenerative FTD Tauopathies. It is not known how changing the 3R/4R Tau ratio at a later stage in the adult animal will affect Tau pathogenesis.
Serine-threonine directed phosphorylation regulates the microtubule binding ability of Tau. Hyperphosphorylation promotes detachment of Tau from microtubules. Other post translational modifications of Tau have been described; however the significance of these is unclear. Phosphorylation of Tau is also developmentally regulated with higher phosphorylation in fetal tissues and much lower phosphorylation in the adult. One characteristic of neurodegenerative disorders is aberrantly increased Tau phosphorylation. The microtubule network is involved in many important processes within the cell including structural integrity needed for maintaining morphology of cells and operating transport machinery. Since binding of Tau to microtubules stabilizes microtubules, Tau is likely to be a key mediator of some of these processes and disruption of normal Tau in neurodegenerative diseases may disrupt some of these key cellular processes. One of the early indicators that Tau may be important in neurodegenerative syndromes was the recognition that Tau is a key component of neurofibrillary inclusions in Alzheimer's disease. In fact, neurofibrillary inclusions are aggregates of hyperphosphorylated Tau protein. Along with amyloid beta containing plaques, neurofibrillary inclusions are a hallmark of Alzheimer's disease and correlate significantly with cognitive impairment. 95% of Tau accumulations in AD are found in neuronal processes and is termed neuritic dystrophy. The process(es) whereby this microtubule associated protein becomes disengaged from microtubules and forms accumulations of proteins and how this relates to neuronal toxicity is not well understood.
Neuronal Tau inclusions are a pathological characteristic of not only Alzheimer's disease, but also a subset of Frontotemporal dementia (FTD), PSP, and CBD. The link between Tau and neurodegeneration was solidified by the discovery that mutations in the Tau gene cause a subset of FTD. These genetic data have also highlighted the importance of the 3R:4R ratio of Tau. Many of the Tau mutations that cause FTD lead to a change in Tau splicing which leads to preferential inclusion of exon 10, and thus to increased 4R Tau. The overall Tau levels are normal. Whether the Tau isoform change or the amino acid change or both cause neurodegeneration remains unknown. Recent data suggest that PSP may also be associated with an increased 4R:3R Tau ratio.
To help understand the influence of Tau ratios on neurodegeneration, a mouse model based on one of the splicing Tau mutations (N279K) has been generated using a minigene that includes the Tau promoter and the flanking intronic sequences of exon 10. As in humans, these mice demonstrate increased levels of 4R Tau compared with transgenics expressing WT Tau and develop behavioral and motor abnormalities as well as accumulations of aggregated Tau in the brain and spinal cord.
Tau protein has been associated with multiple diseases of the brain including Alzheimer's disease, FTD, PSP, CBD, dementia pugilistica, parkinsonism linked to chromosome, Lytico-Bodig disease, tangle-predominant dementia, ganglioglioma, gangliocytoma, meningioangiomatosis, subacute sclerosing panencephalitis, lead encephalopathy, tuberous sclerosis, Hallervorden-Spatz disease, Pick's disease, argyrophilic grain disease, corticobasal degeneration or frontotemporal lobar degeneration and others. Tau-associated disorders such as AD are the most common cause of dementia in the elderly. AD affects an estimated 15 million people worldwide and 40% of the population above 85 years of age. AD is characterized by two pathological hallmarks: Tau neurofibrillary inclusions (NFT) and amyloid-β (Aβ) plaques.
There is currently a lack of acceptable options for treating such neurodegenerative diseases. It is therefore an object herein to provide methods for the treatment of such diseases.